1. Field of the Invention
The present invention relates to a method and system for feeding electrical power to a network termination device required for a DSL (Digital Subscriber Line) service.
2. Description of the Related Art
With the wide use of digital switching systems, an increasing number of DSL network termination devices for DSL service are installed at subscriber sites. The DSL technology dramatically increases the digital capacity of ordinary subscriber lines into the subscriber site, that is, home or office. DSL includes two types of usage: Asymmetric DSL (ADSL) and Symmetric DSL (SDSL or HDSL). ADSL is used for Internet access, where fast downstream Is required, but slow upstream is acceptable. SDSL is designed for short haul connections that require high speed in both directions.
In the case where the electrical power fed into a DSL network termination device fails or drops to an unacceptable voltage level, it is necessary to feed necessary power to the DSL network termination device to ensure at least telephone communication.
As a conventional power feeding method, a large-capacity battery or capacitor is installed in the DSL network termination device and, in the event of a problem, it is used as backup power. However, such a large-capacity battery can provide necessary power for only a limited time period.
As another conventional method, central office-side line termination equipment feeds the subscriber-side DSL network termination device with electrical power through the subscriber line. In this case, DC power is superimposed on a data/voice signal. Therefore, as disclosed in Japanese Patent Application Unexamined Publication No. 6-311206, a power splitting filter for extracting DC power from the subscriber line is needed in a digital service unit (DSU).
Such a power feed system from network to subscriber, however, causes high power losses due to the high impedance of a subscriber line, reducing power-feeding efficiency.